Support content elements in React adapter

[platosha]

Add support for adding Flow components as children to ReactAdapterComponent.

Implementation idea:

• add useContent(name: string) ReactAdapterElement hook, supports rendering a DOM placeholder in React
• add the corresponding protected Element getContentElement(String name) Java ReactAdapterComponent method, that creates and returns the named container element.

Prototype: https://github.com/vaadin/flow-hilla-hybrid-example/tree/proto/flow-react-children

The first implementation could use two DOM wrapper elements, each with style "display: contents;".